The Response of Giant Stars To Dynamical-Timescale Mass Loss

We study the response of giant stars to mass loss. One-dimensional simulations of red and asymptotic giant branch stars with mass loss rates from $10^{-3}$ up to a few \msun/yr show in no case any significant radius increase. The largest radius increase of 0.2% was found in the case with the lowest mass loss rate. For dynamical-timescale mass loss rates, that may be encountered during a common envelope phase, the evolution is not adiabatic. The superadiabatic outer layer of the giant's envelope has a local thermal timescale comparable to the dynamical timescale. Therefore, this layer has enough time to readjust thermally. Moreover, the giant star is driven out of hydrostatic equilibrium and evolves dynamically. In these cases no increase of the stellar radius with respect to its initial value is found. If the mass loss rate is high enough, the superadiabaticity of the outer layer is lost progressively and a radiative zone forms due to a combination of thermal and dynamical readjustment. Conditions for unstable mass transfer based on adiabatic mass loss models that predict a significant radius increase, may need to be re-evaluated.Comment: 9 pages, 11 figures, accepted for publication in ApJ. Minor changes since previous versio

It's EZ to Evolve ZAMS Stars: A Program Derived from Eggleton's Stellar Evolution Code

"Evolve ZAMS", "EZ" for short, is derived from Peter Eggleton's stellar evolution program. The core of EZ is a stripped down, rewritten version of a subset of Eggleton's code, specialized to handle single star evolution from the zero-age main sequence until forced to stop by an event such as a helium flash or a crystallizing core. The procedure and data interfaces to the program are designed to be easy to use while still providing a wide range of function. EZ is written in Fortran 95 following current programming practices and can be downloaded from http://theory.kitp.ucsb.edu/~paxton/.Comment: 2 pages. To appear in PASP. Download tar file with source code, data, and instructions for building EZ from http://theory.kitp.ucsb.edu/~paxton/ -- website has more information and pdf's for many plots of stellar evolutio

Modules for Experiments in Stellar Astrophysics (MESA)

Stellar physics and evolution calculations enable a broad range of research in astrophysics. Modules for Experiments in Stellar Astrophysics (MESA) is a suite of open source libraries for a wide range of applications in computational stellar astrophysics. A newly designed 1-D stellar evolution module, MESA star, combines many of the numerical and physics modules for simulations of a wide range of stellar evolution scenarios ranging from very-low mass to massive stars, including advanced evolutionary phases. MESA star solves the fully coupled structure and composition equations simultaneously. It uses adaptive mesh refinement and sophisticated timestep controls, and supports shared memory parallelism based on OpenMP. Independently usable modules provide equation of state, opacity, nuclear reaction rates, and atmosphere boundary conditions. Each module is constructed as a separate Fortran 95 library with its own public interface. Examples include comparisons to other codes and show evolutionary tracks of very low mass stars, brown dwarfs, and gas giant planets; the complete evolution of a 1 Msun star from the pre-main sequence to a cooling white dwarf; the Solar sound speed profile; the evolution of intermediate mass stars through the thermal pulses on the He-shell burning AGB phase; the interior structure of slowly pulsating B Stars and Beta Cepheids; evolutionary tracks of massive stars from the pre-main sequence to the onset of core collapse; stars undergoing Roche lobe overflow; and accretion onto a neutron star. Instructions for downloading and installing MESA can be found on the project web site (http://mesa.sourceforge.net/).Comment: 110 pages, 39 figures; submitted to ApJS; visit the MESA website at http://mesa.sourceforge.ne